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<h1>Hydrogen, H<sub>2</sub></h1>

Hydrogen (H<sub>2</sub>) is the most abundant element in the universe and forms diatomic molecules under standard conditions. In the realm of optics, hydrogen is commonly encountered in the form of gas cells that are used for Raman spectroscopy and as a reference in various types of spectroscopic analysis. Hydrogen's transparency over a broad range of wavelengths makes it useful in applications that require minimal interaction with light, although it's generally not used in its elemental form for robust optical components due to its reactivity and the challenges associated with containing it. Hydrogen gas cells can also serve as frequency standards in metrology.

<h2>External links</h2>
<ul>
<li><a href="https://en.wikipedia.org/wiki/Hydrogen">Hydrogen - Wikipedia</a></li>
<li><a href="https://www.webelements.com/hydrogen/">Hydrogen - WebElements</a></li>
<li><a href="https://webbook.nist.gov/cgi/cbook.cgi?ID=C1333740">Hydrogen - NIST Cemistry WebBook</a></li>
</ul>
